34 INITIAL CAUSES. 



Processes as causes. — In the strictest sense there is perhaps but a single 

 universal initial cause of succession, namely, a bare area in which pioneers 

 can establish themselves. It is somewhat confusing, if not illogical, to term 

 a passive area a cause, and in consequence the term is referred back to the 

 active processes and agents which produce the bare area. The latter is the 

 initial fact in so far as the development is concerned, but its cause leaves a 

 directive result in the form of the physical factors which characterize the new 

 area. It must also be recognized that succession does not necessarily occur 

 in every bare area. Two other prerequisites must also be met: there must 

 be an adjacent or accessible plant population and the physical conditions of 

 the habitat must permit ecesis. These are almost universal concomitants of 

 bare habitats, the rare exceptions occurring only in the salt-incrusted beds of 

 old lakes in arid regions and perhaps in ice-bound polar areas. Further excep- 

 tions are naturally furnished by wave or tide swept shores and rocks, but 

 these are hardly to be regarded as bare areas. 



Change of conditions. — In a denuded area, moreover, succession proper 

 can not occur unless the physical conditions are essentially changed. This is 

 especially true when the adjacent population is mobile. In such cases a short 

 apparent succession may result, owing to differences in rate of germination 

 and growth, but in some cases, at least, the migrants all enter the same year. 

 Thus in certain lodgepole pine burns of the Rocky Mountains, firegrass, fire- 

 weed, aspen, and lodgepole pine appear together the first year after the fire, 

 but there is an apparent sere of three or four stages, due merely to differences 

 in rate of growth and consequent dominance. A wholly different example is 

 found in certain deserts with one or two distinct rainy seasons, characterized 

 by annuals. This is typical of the deserts of Arizona and adjacent parts of 

 Mexico and California, in which communities of summer and winter annuals 

 appear each season, only to disappear before the subsequent drouth. These 

 represent the pioneer stage of a succession which can not develop further 

 because of extreme conditions. 



A bare area, then, must not merely permit the invasion of an adjacent popu- 

 lation; it must also present conditions that are essentially different if succes- 

 sion is to result. This is typically the case, since the conditions of formation 

 of new soil differentiate it from the habitats of neighboring communities, while 

 the removal of the plant covering materially modifies the habitat, with rare 

 exceptions. As a consequence, an initiating process must accomplish two 

 results: it must produce a bare area capable of ecesis, and it must furnish it 

 with physical factors essentially different, in quantity at least, from the adja- 

 cent areas. In short, a bare area, whether new or denuded, to be capable 

 of succession must be more extreme than the surrounding habitats. This 

 departure from the mean is best seen in the denuding of climax formations, in 

 which case the climatic control is disturbed. In the grass formation of central 

 Nebraska denudation by wind erosion produces a departure toward the xero- 

 phytic extreme, and by flooding, one toward the hydrophytic extreme. 



Fundamental nature of water-content. — In the vast majority of bare areas 

 the departure has to do with water-content, usually its quantity but often its 

 quality, as in saline and acid areas. Light is less frequently concerned, while 

 changes of other efficient factors — temperature, nutrients, and aeration — 

 appear to be subordinate. In all cases the production of a more extreme con- 



